Microfluidic devices have been designed that are useful in performing high throughput assays useful for biological and chemical screening experiments. Both glass and polymer microfluidic devices comprising microfluidic channels and microfluidic wells are now available. For example, polymer microfluidic devices are provided in PCT application WO 98/46438, xe2x80x9cControlled Fluid Transport in Microfabricated Polymeric Substrates,xe2x80x9d by Parce et al., and glass devices are set forth in a number of publications and patents set forth herein.
Continuous flow microfluidic systems are set forth in, e.g., published PCT application WO 98/00231, by Parce et al. These devices are useful, for example, in screening large numbers of different compounds for their effects on a variety of chemical and biochemical systems. The devices include a series of channels fabricated on or within the devices. The devices also can include reservoirs, fluidly connected to the channels, that can be used to introduce a number of test compounds into the sample channels and thus perform the assays. Interfacing mechanisms, such as electropipettors, can be incorporated into these high-throughput systems for transporting samples into wells or microfluidic channels. See, e.g., xe2x80x9cElectropipettor and Compensation Means for Electrophoretic Bias,xe2x80x9d U.S. Pat. No. 5,799,868, by Parce et al.
Microfluidic systems for fast, accurate and low cost electrophoretic analysis of materials in the fields of chemistry, biochemistry, biotechnology, molecular biology and numerous other fields, are described in U.S. Pat. No. 5,699,157 by Parce et al. Techniques for transporting materials through microfluidic channels using electrokinetic forces were described in xe2x80x9cElectropipettor and Compensation Means for Electrophoretic Bias,xe2x80x9d U.S. Pat. No. 5,799,868, by Parce et al.
Movement of material through microfluidic channels was further described in xe2x80x9cVariable Control of Electroosmotic and/or Electrophoretic Forces within a Fluid Containing Structure Via Electrical Forces,xe2x80x9d U.S. Pat. No. 5,800,690, by Chow et al. In this patent, various power supplies, such as a time-multiplexed power supplies that vary the voltage on the system, are described that are used to provide control over the fluid movement in a microfluidic device.
Electroosmotic pressure flow has also been described to provide other ways to modulate microfluidic flow rates. For example, these methods can involve providing an effective zwitterionic compound in the fluid containing the material to be transported. See, e.g., Published PCT application, WO 98/45929 by Nikiforov at el. Additionally, Published PCT application, WO 98/56956 by Kopf-Sill et al. provides methods of correcting for variable velocity in microfluidic devices.
Channel dimensions have also been varied to provide further control over the movement of fluid through the channels, such as in xe2x80x9cMicrofluidic Systems Incorporating Varied Channel Dimensions,xe2x80x9d See, e.g., U.S. Pat. No. 5,842,787, by Kopf-Sill et al.
Although corrections can be made for variable velocities, see, e.g., Published PCT application, WO 98/56956, by Kopf-Sill et al., it is advantageous to be able to rapidly and easily modulate the velocity or flow rate of a component in a microfluidic device. There exists a need for high throughput screening methods, and associated equipment and devices, that are capable of performing repeated, accurate assay, operating at very small volumes and at regulated and/or continuous flow rates. These assays are particularly useful for high throughput screening, as well as for a variety of research applications.
The present invention meets these and a variety of other needs. In particular, the present invention provides novel methods and apparatuses for performing assays with continuous or discontinuous flow rates, as well as other apparatus methods and integrated systems, which will be apparent upon complete review of the disclosure.
This invention provides methods, devices and systems for sustaining and/or modulating and/or measuring flow rates in a microfluidic system by modulating pressure downstream from the region or material of interest. In accordance with the invention, flow rates are modulated or regulated to provide continuous or discontinuous flow by a variety of means. For example, an absorbent material such as an absorbent gel, absorbent polymer material or cellulose containing material is optionally placed downstream from the region or material of interest. Alternatively, or additionally, electrokinetic or pressure based injection or withdrawal of materials into or from the system downstream of the material or region of interest may be used to modulate upstream flow rates. For example, a wick (which can be pre-wetted, dry or wetted in position in contact with a microfluidic system) can act by capillary action to draw material through channels or wells in which it is placed in fluidic contact. Alternatively, or additionally, a volume of liquid is optionally injected or withdrawn downstream of the material or region of interest and the flow rate modulated by creating a pressure differential at the site of injection. Microfluidic devices are provided that contain absorbent materials in particular wells or that have particular wells located to serve as microfluidic injection sites.
In one embodiment, the invention provides a method of modulating the flow rate of material in a microfluidic channel system by modulating pressure downstream of the material, thereby increasing or decreasing flow rate of the material in the channel. Pressure modulation is optionally achieved by placing an absorbent material, such as a wick, in a microfluidic well, by electrokinetic injection, by creating a pressure differential, or by a combination of these three methods.
The absorbent material used to modulate pressure in a microfluidic system is placed. e.g., within a well, such as a waste well, or at the junction of a well and a channel. It can extend beyond the top of the well or remain within the well. The absorbent material is, e.g., a solid, porous, gel, or polymeric material. It is optionally, e.g, a high salt fluid, a thermoplastic polymer (e.g., which is porous or sintered) a porous plastic, or a polyolefin resin. Typically, the absorbent material will be a cellulosic material such as a piece of paper, e.g., a Kimwipe, paper towel, cellulose membrane, nylon membrane, Whatman(trademark) filter, blotting paper, filter paper, cloth or fibrous material, or a polymer, such as dried cross-linked polyacrylamide, or a porous or sintered polymer such as a porous or scintered polyethylene, polypropylene, polyvinylidene fluoride, ethylene-vinyl acetate, polytetrafluoroethylene, stryene-acrylonitrile, polysulfone, polycarbonate, or polyhthalate polymer.
The invention also provides a method for modulating flow rate of material in a microfluidic system by electrokinetic injection of a second material downstream of the first material or region of interest. The flow rate is monitored before injection and/or after injection so that it is sustained at a certain level and controlled.
The invention provides methods of monitoring flow rates by detecting a signal from the material in the channel and measuring the duration and amplitude of a signal that is detected by monitoring fluorescence, phosphorescence, radioactivity, pH, or charge.
In another embodiment, the invention provides a method for determining velocity of a particle in a microfluidic channel system by detecting a signal from the particle for a period of time. The signal amplitude corresponds to the number of particles, and the duration corresponds to the velocity of the particle. Once determined, the velocity is optionally modulated or made constant by electrokinetic injection or by use of an absorbent material such as a wick.
In another embodiment, the invention provides for microfluidic devices that contain wicks or other absorbent materials for use in modulating the flow rate of materials in the device. The devices are made to accommodate flow rate control by wicking or other capillary forces, as described above, by electrokinetic injection, pressure differential or a combination of flow rate control elements. A microfluidic system optionally includes a computer and software for simultaneous or sequential monitoring or control over flow rates, as well as analysis.